1. Field of the Invention
The invention relates to computer controlled graphics machines, and in particular to wide thermal printers.
2. Description of the Prior Art
Conventional thermal printers used for printing narrow text or drawings include a device for driving thermal paper, and a print head having an active strip of resistances disposed transversely across the paper. The head is pressed against the paper and a rotating platen provides backing pressure. The paper is driven by means of a motorized drum with a pressure wheel pressing the paper against the drum. The platen for providing the print head backing may also be used as the paper-driving drum.
The thermal print head strips which are currently available in the trade are relatively short, being generally less than 32 cm. long. A known technique for obtaining large format drawings is to use a plurality of print heads which are offset relative to one another to cover the entire width of the paper. Thus, to obtain a transverse line of points, a first transverse segment is printed using a first head, the paper is advanced until the first printed segment is aligned with a second head, and a second segment is then printed with the second head. Such a technique is described in U.S. Pat. No. 4,660,052 (Kaiya). In this technique, the motorized drum is driven by a stepper motor to synchronize the instant at which the successive heads write with the motion of the paper drive drum.
One disadvantage of this technique is that it leaves line defects at the junctions between different print heads, which defects are increasingly undesirable, given the quality criteria expected of modern graphics machines. Because of manufacturing tolerances in the mechanical components of the machine, and because of the elasticity of the paper, the paper does not move along an exactly rectilinear trajectory, nor does it reproduce the motion of the driving drum stepper motor exactly. Thus, even if each machine is individually adjusted to eliminate junction defects, on average, it is still not possible with machines of the type described in the above-mentioned Kaiya patent to eliminate fluctuations in such defects by performing such adjustment.
This invention provides a multihead thermal printer in which drawing imperfections at the junctions between heads are considerably reduced. It also provides a wide thermal printer of relatively low cost, which is easy to use, and which provides high-quality marking.